A meander is a sinuous curve or bend that develops in a river channel as it flows across a landscape. These characteristic snake-like patterns are most often found in the middle and lower courses of a river where the terrain is relatively flat and the river’s gradient is gentle. Meanders are formed by the continuous interplay between erosion and deposition of sediment, making the river a dynamic system that continuously reshapes its path and the surrounding floodplain.
How Meanders Form
Meanders begin with an uneven distribution of water velocity within a relatively straight river channel. A slight irregularity, such as a sediment bar, deflects the flow of water. This deflection causes the line of fastest water flow, known as the thalweg, to swing toward one bank, making the water move faster on the outside of the resulting curve.
Water flowing around a bend generates a phenomenon called helicoidal flow, which is a corkscrew-like secondary current. This rotational flow moves water and eroded sediment from the outer bank, where velocity is highest, across the channel bed and toward the inner bank. The faster water velocity on the outside bend increases the erosive power, while the slower velocity on the inside reduces the water’s capacity to transport its sediment load.
This differential flow initiates a positive feedback loop that intensifies the bend over time. Erosion continually undercuts the outer bank, deepening the channel, while sediment simultaneously accumulates on the inner bank.
Defining the Key Components
Every fully developed meander features two distinct, opposing landforms that result from the differential flow dynamics. The outer, concave bank of the curve is known as the cut bank, or river cliff. This bank is steep, often nearly vertical, and is the area where the river’s energy is highest, leading to continuous erosion and undercutting.
Directly opposite the cut bank, on the inner, convex side of the meander, is the point bar. This feature is a depositional accumulation of alluvium, formed where the water velocity is lowest. Point bars have a gentle, slip-off slope that gradually rises from the channel bed toward the inner bank.
As the meander migrates, the cut bank erodes the land, and the point bar extends into the evacuated space. This coupled action ensures the river channel maintains a relatively constant width.
The Evolution of a Meandering River
Meanders are constantly migrating across the floodplain. The continued erosion on the cut bank and deposition on the point bar causes the entire meander loop to shift both laterally and downstream. This continuous movement creates a complex pattern of overlapping and abandoned channels visible across the river valley.
As two successive meanders migrate, they eventually approach each other, narrowing the strip of land between their cut banks, which is called the meander neck. When the meander neck becomes sufficiently narrow, the river may breach the thin land barrier. This event, known as a meander cutoff, creates a new, straighter, and more efficient path for the main river flow.
Once the cutoff occurs, the flow through the abandoned meander loop dramatically decreases. Sediment is quickly deposited at the entrance and exit points of the old channel, effectively sealing it off from the main river. This isolated, crescent-shaped body of water that remains is termed an oxbow lake.
Over many years, the standing water in the oxbow lake is slowly filled with fine silt and organic material deposited during subsequent floods. The lake eventually transforms into a marshy area before fully drying out, leaving behind a distinctive crescent-shaped depression on the floodplain known as a meander scar.